Power converters, such as switched-mode power supplies (SMPS), convert input current and input voltage from a source to a different output current and output voltage at a load. Multiple power converters, each having an independent output power, can be electrically connected together in series and parallel arrays to serve a range of applications, including multi-cell battery packs, lighting arrays, computing systems with multiple processors, heating arrays, and electric motors. Such power converter arrays, however, can become unstable without design consideration of the interactions among the multiple power converters. Moreover, these arrays do not enable a dynamic assignment of desired proportioning of power to and from the power converters in the array.
In one application, a power converter array is used for a multi-cell battery pack. Battery packs, or arrangements of multiple energy storage cells coupled together, are used as power sources in a host of devices. The devices include all-electric vehicles, hybrid electric vehicles, portable electronic devices, military applications, medical devices, back-up power and distributed energy storage systems in residential and business locations. Improvements in underlying electrochemistry have yielded batteries with improved performance characteristics, for example, the Li-ion battery. However, even where multiple energy storage cells are intended to be the same in structure and performance characteristics, there are differences among individual energy storage cells. Even with state-of-the-art manufacturing, energy storage cells are inherently dissimilar and demonstrate variations in capacity, lifetime, rates of charge/discharge, and other inter-related properties. For example, a battery pack containing a collection of individual cells may exhibit cell-to-cell differences in energy storage capacity of 2-3% when new, and the variation of energy storage capacity among individual cells tends to increase over time (e.g., as the battery pack ages and is charged and discharged multiple times). Since individual cells of a conventional battery pack may be electrically connected in series to form a series string, the overall performance of the battery pack is degraded by the performance of the weakest cell in the series. For example, with conventional pack architectures, in a series string of cells, the first cell that becomes discharged during use negatively limits the discharge capability of other cells in the series.
Conventional approaches have attempted to address the aforementioned problems and improve the performance of battery packs by providing charge balancing, i.e., electronic circuitry intended to equalize cell voltages or states of charge. Such charge-balancing systems include electrical switches and other electrical elements (e.g., resistors, capacitors, inductors) present at each cell, or grouping of cells, of the battery pack. In such charge balancing systems, resistors may be intermittently connected in parallel with cells in a coordinated manner to equalize cell charging voltages by shunting excess current. In other charge balancing systems, capacitors or inductors are intermittently connected in parallel with cells, such that charge can be transferred from relatively-high-voltage cells to relatively-low-voltage cells. In this manner, performance variations among cells are partially managed so that cells in the battery pack converge toward a desired voltage or state of charge.
Conventional switched-resistor, switched-capacitor, and switched-inductor battery management system architectures provide only partial solutions to the problem of performance variation among cells in multi-cell packs. These battery management systems have only a limited ability to accommodate variations in cell capacity, lifetime, maximum rates of charge/discharge, and other properties of multi-cell packs. Moreover, conventional battery management systems, while compensating for usage performance, may actually reduce the useable lifetime of cells in a battery pack. As a result, in conventional battery packs, useful lifetime is diminished and is typically limited by the weakest cells in the pack.
A prior method of managing the differences performance in cells is by charging and discharging each individual cell in a battery pack at a unique rate so that all cells in the battery pack are at the same proximate state of charge at any given moment.
A prior system to realize independent charge and discharge currents is a parallel converter arrangement. In this arrangement, each electrochemical cell is coupled to an independent power converter that is connected directly to the charging and load bus. The combination of each cell and the corresponding power converter draws a fraction of charge current during the charge phase, and delivers a fraction of load current during the load phase. For each charging or discharging phase, a control mechanism distributes two coefficients to the power converters—a proportionality coefficient that only needs to change at a low rate sufficient to maintain cell state-of-charge matching across cells in the battery pack; and a scaling coefficient that tracks real-time power response requirements of the application. The proportionality and scaling coefficients can be arranged to represent either current or power. The output power of the DC-DC converter on the bus vs. the power across the cell is the efficiency of the DC-DC converter in the charge direction, and the reciprocal of the DC-DC converter efficiency in the discharge direction.
The disadvantages of the parallel converter approach include the need for a power converter coupled to each cell that can tolerate the entire voltage of the charge/load bus. In situations where the charge/load bus operating voltage is many times that of each cell, conversion efficiency is limited, and implementation is costly, typically requiring a step-down/step-up transformer, one or more switching transistors on either side of the transformer, as well as a bias power supply for the bus side control circuits. Efficient operation and economic implementation further limit the range of bus voltages that a given DC-DC converter implementation can accommodate. For example, a DC-DC converter suitable for a 50V bus would require a different transformer and bus-side power electronics than a DC-DC converter suitable to a 300V bus.